A teenage girl was killed as troops opened fire on unarmed protesters in the Honduran capital on Saturday, after the government declared a 10-day curfew and suspended constitutional rights in an attempt to contain an escalating political crisis fuelled by evidence of electoral fraud.

According to witnesses, Kimberly Dayana Fonseca, 19, was shot dead in Tegucigalpa in the early hours of Saturday morning by military police – members of a huge force loyal to the rightwing government of Juan Orlando Hernández, who is accused of meddling in the vote count after last Sunday’s election in an attempt to cling to power.

There were reports of mass detentions and serious injuries overnight after the government deployed troops across the country in what many fear is a return to autocratic rule. At least four people were confirmed dead.

Six days after the election, the winner of the presidential race has still not been declared by the beleaguered electoral commission (TSE), which is controlled by Hernández’s National party.

The opposition Alliance leader, Salvador Nasralla, was five points ahead until a spate of irregularities including mysterious delays in rural votes and computer glitches saw his seemingly insurmountable lead overturned.

“The fraud can no longer just be called fraud,” Eugenio Sosa, a sociologist and political analyst, told the Guardian. “This is a type of electoral coup against the president-elect, Salvador Nasralla.”

The election debacle has plunged the Central American country of 8.5 million people into its worst political crisis since a 2009 military-backed coup which unleashed a violent crackdown against social and political activists.

Relatives of Kimberly Fonseca who was shot during a protest mourn next to her coffin in Tegucigalpa. Photograph: Edgard Garrido/Reuters

The current crisis is closely linked to the consolidation of power by Hernández and his allies since the coup, which has given the National party control over Congress, the judiciary and the armed forces. Hernández used a controversial ruling by friendly judges in the supreme court to justify his bid for a second term in power, despite the constitution prohibiting re-election of sitting or former leaders.

Military police units were patrolling the streets of Tegucigalpa, where no unrest had been reported by Saturday afternoon. The curfew, which began on Friday at 11pm local time with very little warning, suspends the right to free movement from 6pm to 6am and allows security forces to detain anyone breaking the curfew or who is “in some way suspected of causing damage to people or their property”.

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The opposition Alliance claimed looting and violence seen in some cities was prompted by government-aligned provocateurs tasked with generating chaos in order to justify state repression.

Human rights groups fear that a new terror law, approved weeks before the election, will be used to quell dissent. Thousands are expected to demonstrate across the country on Sunday morning, after opposition leaders issued a call for peaceful mobilizations in public parks and squares.

Meanwhile, cracks appear to be growing within the electoral commission. TSE magistrate Marco Ramiro Lobo told the Guardian the delayed rural votes and computer failures – the main system and backup both allegedly failed – must be investigated as “the tribunal president on Monday gave the order to stop counting for 10 to 12 hours”.

Alliance leaders are locked in negotiations with the TSE over an 11-point list of demands.

“The Alliance’s demands for a transparent recount should be met … otherwise this will go on for days and call into question the legitimacy of the process,” said Lobo.